I had been running into issues with the new SAS keeping on a heading last night, so decided to run some tests today. Clean install from the store, no mods, running on Windows. I've never had a joystick installed, and kept my hands off the keyboard/mouse the whole time. Ship has Mk1 command pod, remote guidance unit (so I wouldn't have to kill Jeb), and inline stabilizer. It has no tailfins and uses the LV-T30 engine, so no gimballing. Any control (as I understand SAS) will therefore have to come from rotation wheels and RCS (when enabled). It also has two SRBs attached so it can reach orbit (one on each side of the main payload, in an east-west orientation). I launched 10 times with SAS disabled and 10 times with SAS enabled. I took my final heading/elevation readings when the SRBs cut out, as the SAS was always able to bring me back vertical after they detached. With SAS disabled, the rocket tended to go either west (270°, 284°, 234°, 237°) or southeast (133°, 109°, 142°, 121°, 156°, 114°). Final elevations ranged from 75-87°, and didn't seem to follow a pattern with direction. This may have something to do with the SRBs being on the east and west sides of the rocket. With SAS enabled (but RCS disabled), the rocket tended to go west (7/10 trials - 262°, 265°, 265°, 282°, 276°, 259°, 264°). However, in two of the trials, SAS decided to take me southeast (119°, 96°). Only in one trial did I really manage to go straight up (just a tiny bit east, elevation at 89° or better). All other elevations ranged from 84-87°. In addition, I launched 5 times with both SAS and RCS enabled. In four of the tests, I ended on a heading of 270° and 88° elevation. RCS remaining from my 100-unit tank ranged from 98.36-99.13. In the other test I went straight up, with no detectable heading, 90° elevation, and 99.51 RCS remaining. Overall, it seems to be less of over/under-correcting and more of a westward bias by the SAS. It is physically possible to fly this thing vertically (as shown by two of the tests, and I've done this manually too). It's possible that this has to do with the aging speed of my computer - SAS might not be able to check all the required parameters before it's required to respond, and those small errors accumulate over time.
